Passively Q-switched laser apparatuses (hereinafter referred to simply as “laser apparatuses”) are for generating pulse light and utilized in spectroscopic measurements, geometrical measurements, and nonlinear crystal excitations, etc. In such laser apparatuses, a saturable absorber is arranged together with a laser medium between a pair of mirrors (reflecting means) that constitute an optical resonator. In this arrangement, when light emitted from the laser medium being excited enters the saturable absorber, the emitted light is absorbed by the saturable absorber. The electron density in the excited level of the saturable absorber increases gradually with the absorption of the emitted light, and when the excited level is filled up, i.e., the electron density in the excited level is saturated at some point in time, the saturable absorber becomes transparent. At this time, the Q-value of the optical resonator increases drastically to result in a laser oscillation and thereby generating pulse light.
Meanwhile, it is desirable that the polarization direction of laser beams from such laser apparatuses is controlled and stabilized for wavelength conversions using nonlinear optical crystals and linear polarization based geometrical measurements, etc. As a method for controlling the polarization direction of laser beams, there have been proposed techniques in which a polarizing element is arranged between a laser medium and a saturable absorber (refer to Non-Patent Documents 1 to 3, for example).
Non-Patent Document 1: A. V. Kir' yanov and V. Aboites, “Enhancing type-II optical second-harmonic generation by the use of a laser beam with a rotating azimuth of polarization”, APPLIED PHYSICS LETTERS, 12 Feb. 2001, Vol. 78, No. 7, pp. 874-876.
Non-Patent Document 2: Alexander V. Kir' yanov and Vicente Aboites, “Second-harmonic generation by Nd3+:YAG/Cr4+:YAG-laser pulses with changing state of polarization”, J. Opt. Soc. Am. B, October 2000, Vol. 17, No. 10, pp. 1657-1664.
Non-Patent Document 3: A. V. Kir' yanov, J. J. Soto-Bernal, and V. J. Pinto-Robledo, “SHG by a Nd3+:YAG/Cr4+:YAG laser pulse with changing-in-time polarization”, Advanced Solid-State Lasers, 2002, Vol. 68, pp. 88-92.